quale pheromonal stimulation, the female 

 would reject her suitor. 



Not all danaine pheromones smell 

 alike, and the human nose can detect some 

 differences in the male perfumes of milk- 

 weed butterfly species. Mostly they smell 

 strong but pleasant to us: some, sweet like 

 chocolate; others, more like pineapple. 

 However, for a more precise identification 

 of the pheromone composition, sophisti- 

 cated technical equipment is needed. Jer- 

 rold Meinwald, of Cornell University, and 

 Stefan Schulz and Wittko Francke, of the 

 University of Hamburg, have analyzed the 

 chemistry of hairpencil extracts taken 

 from many species and found that the 

 pheromones are species-specific bouquets 

 made up of twelve to fifty volatile compo- 



nents, most of which are "unsmellable" by 

 the human nose. 



What is the male telling the female with 

 this fanfare of pheromones? Danaine but- 

 terflies locate one another by sight, so the 

 pheromones caimot be long-range attrac- 

 tants. However, mimicry is very common 

 among these butterflies, so something 

 more than just visual inspection may be 

 necessary to allow members of a species to 

 recognize one another. At close range, the 

 female may use the male's perfume to de- 

 termine which species her suitor belongs 

 to; "Let me smell you so I can know who 

 you are." 



But there appears to be more than spe- 

 cies recognition to the story. Certain 

 chemical compounds are common compo- 



nents in the pheromone bouquets of many 

 danaine species and thus are unlikely to 

 contribute to species specificity. Called di- 

 hydropyrrolizines, these chemicals often 

 make up the largest proportion of the hair- 

 pencil volatiles, with up to 500 ng (a half 

 thousandth of a gram) in a single pair of 

 hairpencils, an enormous amount com- 

 pared with that of pheromones in other in- 

 sects. These chemicals must serve a differ- 

 ent purpose. 



Studies with field-caught male danaines 

 revealed that the amount of dihydropy- 

 rrohzines varies gready from individual to 

 individual. Freshly hatched males possess 

 various other pheromone components but 

 lack dihydropyrrolizines entirely, and as 

 Thomas Pliske and Thomas Eisner, of 



28 Natural History 1/94 



